It is known to utilize magnetron cathode vapor deposition on a substrate to apply a dereflective coating of tin oxide to an optical filter composed of a plurality of coatings on a transparent substrate, at least one of which includes silver. In general, the magnetron cathode sputtering operation is carried out at a predetermined dereflective coating-oxygen partial pressure (E partial pressure) and at a predetermined dereflective coating or sputtering rate (E sputtering or coating rate). The silver coating or the silver coatings can be applied in various ways although preferably in this field these coatings are also applied by magnetron cathode sputtering.
The dereflective coating is preferably applied with the aid of reactive magnetron cathode sputtering and in general this process is carried out, although this is not essential, continuously on a moving substrate in an apparatus provided with gates or curtains through which the substrate can travel.
Generally the substrate is a glass disk or a pane or a synthetic resin plate.
Thin silver coatings are characterized by a high light permeability (translucency) in combination with a high degree of reflection for infrared rays and hence the coated substrates can be utilized effectively as thermal barriers for windows and the like. When such windows are utilized in dwelling or commercial structures, for example, infrared ray incursion is restricted without materially reducing the visible light penetration. Hence windows formed or provided with such coated glass panes or layers can be utilized to assist in air conditioning to thereby reduce the amount of energy which is required to maintain a predetermined temperature in the building.
To improve the selectivity characteristics of silver coatings, it has been found to be advantageous to apply to the surface of the silver coating which is turned away from the substrate a dereflective coating which is effective in the visible light range and is composed of a dielectric material with a reflective index greater than 1.7. It has also been proposed, in connection with such filters, to provide between the transparent substrate and the silver coating a further dielectric layer which acts as a bond-promoting agent and which, when formed as a layer in a thickness corresponding to a quarter wavelength, i.e. as a quarter wavelength coating, it additionally provides a dereflective effect.
Between the substrate and the silver layer, however, in practice, one or more further bond-promoting metal oxide layers can be provided and indeed the dereflective coating on the face of silver layer turned away from the substrate can be provided with one or more additional covering layers or coatings.
Mention may also be made of systems in which between the transparent substrate and the silver coating or between the metal oxide coating and the silver coating, additional metal or metal alloy coatings, e.g. of chromium, nickel, titanium or chromium nickel alloys, can be provided to improve the adhesion to subsequently applied coatings. Naturally such coatings are required to be transparent as well, and thus must be extremely thin coatings with a thickness of one or several atoms, i.e. must be practically of monoatomic thickness (see German patent document-open application DE OS 21 44 242).
Magnetron cathode sputtering or atomization is described in U.S. Pat. No. 4,013,532 and is recognized as a vacuum process which allows high coating rates to be obtained. Such processes permit economically the production of relatively thick dereflective coatings by comparison to the silver coating and hence for the production of dereflective coatings of, tin oxide, for example, reactive magnetron atomization is used. In such systems, rather than effecting the atomization in high vacuum or in an inert gas atmosphere, the gas atmosphere is designed to contain oxygen which reacts with the sputtering metal or metal alloys from the appropriate cathode targets to enable in situ formation of corresponding metal oxides or mixed oxides of the metals of the alloy to thereby generate the dielectric dereflective coating material.
To simplify the following discussion, we may refer below to the E partial pressure, this being defined as the dereflective coating-oxygen partial pressure (in mbar), and to the E coating rate (in A/sec) which, of course, will indicate the dereflective coating deposition rate.
It has been found that conventional processes for the application for the dereflective coating upon a silver coating in a reactive oxygen-containing plasma, the infrared reflection characteristics of the silver coating is reduced. Thus if the infrared reflection of the silver coating is 90% before application of the tin oxide coating, tests after the application of the tin oxide coating show an infrared reflection between 10% and 40%.
While a portion of the infrared reflection loss can be partly compensated by making the silver coating somewhat thicker, this in turn results in a loss of transparency in the visible portion of the spectrum, and hence a decrease in the transparency of the filter. As a result, the effectiveness of the filter is diminished.
As far as we have been able to ascertain, the reason for this change in the characteristics of the silver coating as a result of the application of the dereflective coating is unknown.